This invention was inspired out of the de-embed probe research project that needed a calibrated method to measure the s-parameters of a high impedance probe. Since the inputs to such a probe are its pointed tips and its output is, for example, an SMA (SubMiniature version A) or a BMA (Blind Mate A) connector, it is not possible to use standard vector network analyzer calibration kits. Thus, a special fixture and algorithm method are needed in order to perform the required measurements. Heretofore, probe manufacturers, such as Tektronix, Inc., only measured the response of probes by use of a test fixture terminated in a 50 ohm impedance. However, this method does not incorporate error correction to remove the fixture effects and the measurement errors. What is needed is an apparatus and a method that overcomes these limitations.
The only published articles in the prior art on the subject of measuring s-parameters of a high impedance probe, of which the subject inventors are aware, were published by NIST (National Institute of Standards and Technology). For example, “Wideband Frequency-Domain Characterization of High-Impedance Probes,” (U. Arz, H. C. Reader, P. Kabos, and D. F. Williams), 58th ARFTG Conference Digest, pp. 117-124, Nov. 29-30, 2001, reports an investigation of broadband microwave properties of high-impedance probes designed for on-wafer waveform measurements. The report shows that the standard two-tier characterization method fails, and introduces two new methods of characterization.
Also, “Calibrated waveform measurement with high-impedance probes,” (P. Kabos, H. C. Reader, U. Arz, and D. F. Williams), IEEE Trans. Microwave Theory and Tech., vol. 51, no. 2, pp. 530-535, February 2003 is a report describing the development of an on-wafer waveform calibration technique that combines a frequency-domain mismatch correction to account for the effects of the probe on the measurement with an oscilloscope calibration. The authors state that the mismatch correction is general and can be applied to any type of microwave probe, including scanning and internal-node probes for noninvasive integrated-circuit tests. The authors also state that, for the commercial high-impedance probe we used, their calibration approach allows accurate on-wafer waveform reconstruction for a variety of probe ground configurations.
The above-described NIST calibrations use probe substitution at the end of a transmission line. The substituted probe must have the same footprint as the original RF probe or additional errors will be introduced. The subject inventors note that transmission line end effects are yet another source of error.
“Measuring the invasiveness of high-impedance probes,” (U. Arz, P. Kabos, and D. F. Williams), 7th IEEE Workshop on Signal Propagation on Interconnects., Siena, Italy, May 11-14, 2003, reports the use of on-wafer measurements to characterize the invasiveness of high-impedance probes over a broad frequency range. The authors state that a two-port representation characterizing the invasiveness of the probe can also be determined from a calculation of the probe's load impedance, which is derived from a separate characterization of the high-impedance probe.
The above-described paper uses an indirect approach to quantify probe loading at point on a transmission line using multiple 2-port measurements to construct a Probe Invasiveness Error box. This error box is a 2-port model that is inserted into the transmission line model to account for the probe loading. The paper establishes that a probe's input impedance corresponds to the 2-port invasiveness model inserted into a transmission line but no probe transfer response characteristics are determined.
NIST has developed a frequency-domain method of characterizing high impedance probes suitable for performing noninvasive on-wafer waveform and signal-integrity measurements. In this regard, “Calibrating On-Wafer Probes to the Probe Tips,” (D. F. Williams and R. B. Marks), 40th ARFTG Conference Digest, pp. 136-143, December 1992 states that conventional ANA calibrations are founded upon the assumption that only a single mode exists at the calibration reference plane, both during calibration and measurement. On-wafer probe tip calibrations clearly violate this assumption due to the discontinuity at the probe/wafer contact. This prior art reference describes moving the reference plane of a TRL calibration to the end of the transmission line to calibrate at the probe tip. The subject inventors note that these probes are Microwave-type RF probes, not normally used in oscilloscope probing. The physical construction of this type of probe causes problems when trying to probe in the middle of a transmission line. They are normally used to probe at the end of a line. There have problems with end effects when moving the reference plane to the end of a line and calibrating. The field is not the same as in the center of a transmission line where it is uniform TEM.
“Multiport investigation of the coupling of high-impedance probes” (P. Kabos, U. Arz, and D. F. Williams), IEEE Microwave and Wireless Components Letters, vol. 14, no. 11, pp. 510-512, November 2004, describes an on-wafer measurement technique that combines two-port and three-port frequency-domain mismatch corrections in order to characterize the influence of a high-impedance probe on a device under test. The procedure quantifies the probe's loading of the circuit, and the coupling between the probe and the device.
“LRM Probe-Tip Calibrations with Imperfect Resistors and Lossy Lines,” (D. F. Williams and R. B. Marks), 42nd ARFTG Conference Digest, pp. 32-36, December 1993, received a “Best Paper Award”. This prior art reference states that the line-reflect-match calibration is extended, without significant loss of measurement accuracy, to accommodate imperfect match standards and lossy lines typical of monolithic microwave integrated circuits. The authors characterized the match and line standards using an additional line standard of moderate length.
They stated that their new method provided a practical means of obtaining accurate, wideband calibrations with compact standard sets.
In, “LRM Probe-Tip Calibrations using Nonideal Standards” (Dylan F. Williams, Senior Member, IEEE and Roger B. Marks, Senior Member, IEEE), the authors state that the line-reflect-match calibration is enhanced to accommodate imperfect match standards and lossy lines typical of monolithic microwave integrated circuits. They further stated that they characterized the match and line standards using an additional line standard of moderate length. They claimed that their new method provided a practical means of obtaining accurate, wideband calibrations with compact standard sets, and that without that enhancement, calibration errors due to imperfections in typical standards could be severe.
None of the above-mentioned prior art references recognize the need to duplicate a typical probing environment in which there are no probe pads or transmission line end effects to be compensated.